DecisionTree API change:

* Added 6 static train methods to match Python API, but without default arguments (but with Python default args noted in docs).

Added factory classes for Algo and Impurity, but made private[mllib].

mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala

@@ -32,9 +32,9 @@ import org.apache.spark.mllib.random.{RandomRDDGenerators => RG}
 import org.apache.spark.mllib.recommendation._
 import org.apache.spark.mllib.regression._
 import org.apache.spark.mllib.tree.configuration.Algo._
-import org.apache.spark.mllib.tree.configuration.Strategy
+import org.apache.spark.mllib.tree.configuration.{Algo, Strategy}
 import org.apache.spark.mllib.tree.DecisionTree
-import org.apache.spark.mllib.tree.impurity.{Entropy, Gini, Impurity, Variance}
+import org.apache.spark.mllib.tree.impurity._
 import org.apache.spark.mllib.tree.model.DecisionTreeModel
 import org.apache.spark.mllib.stat.Statistics
 import org.apache.spark.mllib.stat.correlation.CorrelationNames
@@ -498,17 +498,8 @@ class PythonMLLibAPI extends Serializable {
 
     val data = dataBytesJRDD.rdd.map(deserializeLabeledPoint)
 
-    val algo: Algo = algoStr match {
-      case "classification" => Classification
-      case "regression" => Regression
-      case _ => throw new IllegalArgumentException(s"Bad algoStr parameter: $algoStr")
-    }
-    val impurity: Impurity = impurityStr match {
-      case "gini" => Gini
-      case "entropy" => Entropy
-      case "variance" => Variance
-      case _ => throw new IllegalArgumentException(s"Bad impurityStr parameter: $impurityStr")
-    }
+    val algo = Algo.stringToAlgo(algoStr)
+    val impurity = Impurities.stringToImpurity(impurityStr)
 
     val strategy = new Strategy(
       algo = algo,

mllib/src/main/scala/org/apache/spark/mllib/tree/DecisionTree.scala

@@ -17,14 +17,16 @@
 
 package org.apache.spark.mllib.tree
 
+import scala.collection.JavaConverters._
+
 import org.apache.spark.annotation.Experimental
 import org.apache.spark.Logging
 import org.apache.spark.mllib.regression.LabeledPoint
-import org.apache.spark.mllib.tree.configuration.Strategy
+import org.apache.spark.mllib.tree.configuration.{Algo, Strategy}
 import org.apache.spark.mllib.tree.configuration.Algo._
 import org.apache.spark.mllib.tree.configuration.FeatureType._
 import org.apache.spark.mllib.tree.configuration.QuantileStrategy._
-import org.apache.spark.mllib.tree.impurity.Impurity
+import org.apache.spark.mllib.tree.impurity.{Impurities, Gini, Entropy, Impurity}
 import org.apache.spark.mllib.tree.model._
 import org.apache.spark.rdd.RDD
 import org.apache.spark.util.random.XORShiftRandom
@@ -213,10 +215,8 @@ object DecisionTree extends Serializable with Logging {
   }
 
   /**
-   * Method to train a decision tree model where the instances are represented as an RDD of
-   * (label, features) pairs. The method supports binary classification and regression. For the
-   * binary classification, the label for each instance should either be 0 or 1 to denote the two
-   * classes.
+   * Method to train a decision tree model.
+   * The method supports binary and multiclass classification and regression.
    *
    * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
    *              For classification, labels should take values {0, 1, ..., numClasses-1}.
@@ -237,10 +237,8 @@ object DecisionTree extends Serializable with Logging {
   }
 
   /**
-   * Method to train a decision tree model where the instances are represented as an RDD of
-   * (label, features) pairs. The method supports binary classification and regression. For the
-   * binary classification, the label for each instance should either be 0 or 1 to denote the two
-   * classes.
+   * Method to train a decision tree model.
+   * The method supports binary and multiclass classification and regression.
    *
    * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
    *              For classification, labels should take values {0, 1, ..., numClasses-1}.
@@ -263,11 +261,8 @@ object DecisionTree extends Serializable with Logging {
   }
 
   /**
-   * Method to train a decision tree model where the instances are represented as an RDD of
-   * (label, features) pairs. The decision tree method supports binary classification and
-   * regression. For the binary classification, the label for each instance should either be 0 or
-   * 1 to denote the two classes. The method also supports categorical features inputs where the
-   * number of categories can specified using the categoricalFeaturesInfo option.
+   * Method to train a decision tree model.
+   * The method supports binary and multiclass classification and regression.
    *
    * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
    *              For classification, labels should take values {0, 1, ..., numClasses-1}.
@@ -279,11 +274,9 @@ object DecisionTree extends Serializable with Logging {
    * @param numClassesForClassification number of classes for classification. Default value of 2.
    * @param maxBins maximum number of bins used for splitting features
    * @param quantileCalculationStrategy  algorithm for calculating quantiles
-   * @param categoricalFeaturesInfo A map storing information about the categorical variables and
-   *                                the number of discrete values they take. For example,
-   *                                an entry (n -> k) implies the feature n is categorical with k
-   *                                categories 0, 1, 2, ... , k-1. It's important to note that
-   *                                features are zero-indexed.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
    * @return DecisionTreeModel that can be used for prediction
    */
   def train(
@@ -300,32 +293,197 @@ object DecisionTree extends Serializable with Logging {
     new DecisionTree(strategy).train(input)
   }
 
-  // Optional arguments in Python: maxBins
+  /**
+   * Method to train a decision tree model.
+   * The method supports binary and multiclass classification and regression.
+   * This version takes basic types, for consistency with Python API.
+   *
+   * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
+   *              For classification, labels should take values {0, 1, ..., numClasses-1}.
+   *              For regression, labels are real numbers.
+   * @param algo "classification" or "regression"
+   * @param numClassesForClassification number of classes for classification. Default value of 2.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
+   * @param impurity criterion used for information gain calculation
+   * @param maxDepth Maximum depth of the tree.
+   *                 E.g., depth 0 means 1 leaf node; depth 1 means 1 internal node + 2 leaf nodes.
+   * @param maxBins maximum number of bins used for splitting features
+   *                 (default Python value = 100)
+   * @return DecisionTreeModel that can be used for prediction
+   */
   def train(
       input: RDD[LabeledPoint],
-      algo: Algo,
+      algo: String,
       numClassesForClassification: Int,
-      categoricalFeaturesInfo: Map[Int,Int],
-      impurity: Impurity,
+      categoricalFeaturesInfo: Map[Int, Int],
+      impurity: String,
       maxDepth: Int,
-      maxBins: Int): DecisionTreeModel = ???
+      maxBins: Int): DecisionTreeModel = {
+    val algoType = Algo.stringToAlgo(algo)
+    val impurityType = Impurities.stringToImpurity(impurity)
+    train(input, algoType, impurityType, maxDepth, numClassesForClassification, maxBins, Sort,
+      categoricalFeaturesInfo)
+  }
 
-  // Optional arguments in Python: all but input, numClassesForClassification
+  /**
+   * Method to train a decision tree model.
+   * The method supports binary and multiclass classification and regression.
+   * This version takes basic types, for consistency with Python API.
+   * This version is Java-friendly, taking a Java map for categoricalFeaturesInfo.
+   *
+   * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
+   *              For classification, labels should take values {0, 1, ..., numClasses-1}.
+   *              For regression, labels are real numbers.
+   * @param algo "classification" or "regression"
+   * @param numClassesForClassification number of classes for classification. Default value of 2.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
+   * @param impurity criterion used for information gain calculation
+   * @param maxDepth Maximum depth of the tree.
+   *                 E.g., depth 0 means 1 leaf node; depth 1 means 1 internal node + 2 leaf nodes.
+   * @param maxBins maximum number of bins used for splitting features
+   *                 (default Python value = 100)
+   * @return DecisionTreeModel that can be used for prediction
+   */
+  def train(
+      input: RDD[LabeledPoint],
+      algo: String,
+      numClassesForClassification: Int,
+      categoricalFeaturesInfo: java.util.Map[java.lang.Integer, java.lang.Integer],
+      impurity: String,
+      maxDepth: Int,
+      maxBins: Int): DecisionTreeModel = {
+    train(input, algo, numClassesForClassification,
+      categoricalFeaturesInfo.asInstanceOf[java.util.Map[Int, Int]].asScala.toMap,
+      impurity, maxDepth, maxBins)
+  }
+
+  /**
+   * Method to train a decision tree model for binary or multiclass classification.
+   * This version takes basic types, for consistency with Python API.
+   *
+   * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
+   *              Labels should take values {0, 1, ..., numClasses-1}.
+   * @param numClassesForClassification number of classes for classification.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
+   *                                 (default Python value = {}, i.e., no categorical features)
+   * @param impurity criterion used for information gain calculation
+   *                  (default Python value = "gini")
+   * @param maxDepth Maximum depth of the tree.
+   *                 E.g., depth 0 means 1 leaf node; depth 1 means 1 internal node + 2 leaf nodes.
+   *                  (default Python value = 4)
+   * @param maxBins maximum number of bins used for splitting features
+   *                 (default Python value = 100)
+   * @return DecisionTreeModel that can be used for prediction
+   */
   def trainClassifier(
       input: RDD[LabeledPoint],
       numClassesForClassification: Int,
-      categoricalFeaturesInfo: Map[Int,Int],
-      impurity: Impurity,
+      categoricalFeaturesInfo: Map[Int, Int],
+      impurity: String,
+      maxDepth: Int,
+      maxBins: Int): DecisionTreeModel = {
+    train(input, "classification", numClassesForClassification, categoricalFeaturesInfo, impurity,
+      maxDepth, maxBins)
+  }
+
+  /**
+   * Method to train a decision tree model for binary or multiclass classification.
+   * This version takes basic types, for consistency with Python API.
+   * This version is Java-friendly, taking a Java map for categoricalFeaturesInfo.
+   *
+   * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
+   *              Labels should take values {0, 1, ..., numClasses-1}.
+   * @param numClassesForClassification number of classes for classification.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
+   *                                 (default Python value = {}, i.e., no categorical features)
+   * @param impurity criterion used for information gain calculation
+   *                  (default Python value = "gini")
+   * @param maxDepth Maximum depth of the tree.
+   *                 E.g., depth 0 means 1 leaf node; depth 1 means 1 internal node + 2 leaf nodes.
+   *                  (default Python value = 4)
+   * @param maxBins maximum number of bins used for splitting features
+   *                 (default Python value = 100)
+   * @return DecisionTreeModel that can be used for prediction
+   */
+  def trainClassifier(
+      input: RDD[LabeledPoint],
+      numClassesForClassification: Int,
+      categoricalFeaturesInfo: java.util.Map[java.lang.Integer, java.lang.Integer],
+      impurity: String,
+      maxDepth: Int,
+      maxBins: Int): DecisionTreeModel = {
+    trainClassifier(input, numClassesForClassification,
+      categoricalFeaturesInfo.asInstanceOf[java.util.Map[Int, Int]].asScala.toMap,
+      impurity, maxDepth, maxBins)
+  }
+
+  /**
+   * Method to train a decision tree model for regression.
+   * This version takes basic types, for consistency with Python API.
+   *
+   * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
+   *              Labels are real numbers.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
+   *                                 (default Python value = {}, i.e., no categorical features)
+   * @param impurity criterion used for information gain calculation
+   *                  (default Python value = "variance")
+   * @param maxDepth Maximum depth of the tree.
+   *                 E.g., depth 0 means 1 leaf node; depth 1 means 1 internal node + 2 leaf nodes.
+   *                  (default Python value = 4)
+   * @param maxBins maximum number of bins used for splitting features
+   *                 (default Python value = 100)
+   * @return DecisionTreeModel that can be used for prediction
+   */
+  def trainRegressor(
+      input: RDD[LabeledPoint],
+      categoricalFeaturesInfo: Map[Int, Int],
+      impurity: String,
       maxDepth: Int,
-      maxBins: Int): DecisionTreeModel = ???
+      maxBins: Int): DecisionTreeModel = {
+    train(input, "regression", 0, categoricalFeaturesInfo, impurity, maxDepth, maxBins)
+  }
 
-  // Optional arguments in Python: all but input
+  /**
+   * Method to train a decision tree model for regression.
+   * This version takes basic types, for consistency with Python API.
+   * This version is Java-friendly, taking a Java map for categoricalFeaturesInfo.
+   *
+   * @param input Training dataset: RDD of [[org.apache.spark.mllib.regression.LabeledPoint]].
+   *              Labels are real numbers.
+   * @param categoricalFeaturesInfo Map storing arity of categorical features.
+   *                                E.g., an entry (n -> k) indicates that feature n is categorical
+   *                                with k categories indexed from 0: {0, 1, ..., k-1}.
+   *                                 (default Python value = {}, i.e., no categorical features)
+   * @param impurity criterion used for information gain calculation
+   *                  (default Python value = "variance")
+   * @param maxDepth Maximum depth of the tree.
+   *                 E.g., depth 0 means 1 leaf node; depth 1 means 1 internal node + 2 leaf nodes.
+   *                  (default Python value = 4)
+   * @param maxBins maximum number of bins used for splitting features
+   *                 (default Python value = 100)
+   * @return DecisionTreeModel that can be used for prediction
+   */
   def trainRegressor(
       input: RDD[LabeledPoint],
-      categoricalFeaturesInfo: Map[Int,Int],
-      impurity: Impurity,
+      categoricalFeaturesInfo: java.util.Map[java.lang.Integer, java.lang.Integer],
+      impurity: String,
       maxDepth: Int,
-      maxBins: Int): DecisionTreeModel = ???
+      maxBins: Int): DecisionTreeModel = {
+    trainRegressor(input,
+      categoricalFeaturesInfo.asInstanceOf[java.util.Map[Int, Int]].asScala.toMap,
+      impurity, maxDepth, maxBins)
+  }
+
 
   private val InvalidBinIndex = -1
 
@@ -1361,10 +1519,10 @@ object DecisionTree extends Serializable with Logging {
    *   (a) For multiclass classification with a low-arity feature
    *       (i.e., if isMulticlass && isSpaceSufficientForAllCategoricalSplits),
    *       the feature is split based on subsets of categories.
-   *       There are 2^(maxFeatureValue - 1) - 1 splits.
+   *       There are math.pow(2, (maxFeatureValue - 1) - 1) splits.
    *   (b) For regression and binary classification,
    *       and for multiclass classification with a high-arity feature,
-   *       there is one split per category.
+   *
 
    * Categorical case (a) features are called unordered features.
    * Other cases are called ordered features.

mllib/src/main/scala/org/apache/spark/mllib/tree/configuration/Algo.scala

@@ -27,4 +27,10 @@ import org.apache.spark.annotation.Experimental
 object Algo extends Enumeration {
   type Algo = Value
   val Classification, Regression = Value
+
+  private[mllib] def stringToAlgo(name: String): Algo = name match {
+    case "classification" => Classification
+    case "regression" => Regression
+    case _ => throw new IllegalArgumentException(s"Did not recognize Algo name: $name")
+  }
 }

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Impurities.scala

@@ -0,0 +1,32 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.mllib.tree.impurity
+
+/**
+ * Factory class for Impurity types.
+ */
+private[mllib] object Impurities {
+
+  def stringToImpurity(name: String): Impurity = name match {
+    case "gini" => Gini
+    case "entropy" => Entropy
+    case "variance" => Variance
+    case _ => throw new IllegalArgumentException(s"Did not recognize Impurity name: $name")
+  }
+
+}